Here enters a certain limitation in how precisely the expected spectra can be modeled. In the cases discussed so far of thermally driven convection, the basic stellar parameters of surface gravity and effective temperature suffice to formulate a hydrodynamic problem for which ab initio simulations can be carried out. In the case of magnetically affected granulation, however, the unknown quantity of the total magnetic flux on (parts of) the stellar surface enters, which cannot be precisely measured (probably not even for the Sun). In response to transiting active regions, the Sun (seen as a star) varies in irradiance [1]} and in apparent radial velocity [2]}. It has been suggested that the Sun is less active than other G-type stars [3]}, although other arguments exist in the literature. In any case, magnetic activity tends to decrease with age and with slower stellar rotation.
